Guide
Dale–Chall and Spache: readability by familiar words
Most readability formulas estimate difficulty from word length or syllables. The Dale–Chall and Spache formulas take a different view: a word is hard if most young readers do not know it. They count the words in a text that are not on a list of familiar words, combine that with sentence length, and turn it into a grade level. Teachers and librarians have used them for decades to match texts to readers.
The New Dale–Chall formula
Edgar Dale and Jeanne Chall published the formula in 1948 and revised its word list in 1995. The list contains about 3,000 words that at least 80 % of fourth-graders knew. The score is 0.1579 times the percentage of words not on the list, plus 0.0496 times the average sentence length, plus 3.6365 when more than 5 % of words are unfamiliar. A score below 5.0 corresponds to grade 4 and below; 5.0–5.9 grades 5–6; 6.0–6.9 grades 7–8; 7.0–7.9 grades 9–10; 8.0–8.9 grades 11–12; 9.0–9.9 college; 10 or more college graduate.
The Spache formula for early readers
George Spache designed his formula for texts for grades 1 to 3, where Dale–Chall is less precise. The revised 1974 version uses a smaller list of very common words and counts each unfamiliar word only once: grade = 0.121 × words per sentence + 0.082 × percentage of unique unfamiliar words + 0.659. It gives a grade level directly. If the result is above 4, the text is beyond Spache’s range and Dale–Chall is the better measure.
Which words count as familiar
Both formulas let regular forms of listed words count: if “run” is on the list, so are runs, running and runner, and if “happy” is listed, so are happier and happiest. Proper names inside a sentence, such as Paris or Maria, are treated as familiar, as the formulas specify. Hyphenated words count as familiar when every part is on the list.
Using the highlights
Every unfamiliar word is highlighted in the text, and the table ranks them by how often they appear. Words that repeat many times are the best candidates for a simpler synonym or a short explanation the first time they appear. Specialist terms your readers already know can stay; the formula cannot know your audience.
Reliability
The formulas were built on 100-word samples; with fewer than 100 words a note appears. Very short texts, lists and texts full of names give unreliable results.
A worked example
The sample story about a fox at a farm uses only words on the familiar list or regular forms of them, so no word is highlighted and the Dale–Chall score lands well inside the “grade 4 and below” band. Paste the opening of a science article and the percentage of difficult words rises quickly — “photosynthesis”, “experiment” and “hypothesis” are all outside the list — pushing the score into the high-school range.
Revising with the word list
Work from the ranked table: a difficult word used fifteen times affects the score far more than one used once. Replace it where a familiar word works, or keep it and explain it on first use if readers must learn it. Then re-score; small vocabulary changes often move a text down a whole grade band.
Related tools
Get Flesch and other scores with the Readability Checker, try the SMOG Calculator for health information, the LIX Calculator for other languages, or test a real reader with the Cloze Test Maker.
Checked against the current version of the Dale–Chall & Spache Readability on .